Welcome back to my new video series on machine learning with scikit-learn. Last week, we discussed the pros and cons of scikit-learn, showed how to install scikit-learn independently or as part of the Anaconda distribution of Python, walked through the IPython Notebook interface, and covered a few resources for learning Python if you don't already know the language.

This week, we're going to take our first steps in scikit-learn by loading and exploring the famous Iris dataset!

The Iris dataset is made up of 50 samples from three species of Iris. Each sample contains four features: the length and width of the sepals, and the length and width of the petals.

Video #3: Exploring the Iris dataset with scikit-learn

Here's the agenda:

What is the famous Iris dataset, and how does it relate to machine learning?

How do we load the Iris dataset into scikit-learn?

How do we describe a dataset using machine learning terminology?

What are scikit-learn's four key requirements for working with data?

Starting this week, I recommend that you follow along with the code on your own computer. You can type it yourself in the Python environment of your choice, or download the IPython Notebook from my GitHub repository and run it locally.

If you want to challenge yourself and go further than what is shown in the video, try reading in the iris dataset directly from the CSV file rather than loading it from scikit-learn. You could use Python's csv module, the loadtxt() function from NumPy, or the read_csv() function from Pandas. You would ideally end up with the same result as shown in the video, with the features stored in a NumPy array called "X" and the response stored in a NumPy array called "y", each with the proper shape. Feel free to post your code online using a Gist and share a link in the comments section below!

In next week's video, we'll learn about our first machine learning model, train that model in scikit-learn on the iris dataset, and use the model to make predictions. See you again next Wednesday!

Need to get caught up?

Hello Kevin. I am really enjoying the slow, thorough pace of these tutorials. I have a question. I downloaded Anaconda. The home screen of iPhyton (jupyter) includes a bunch of folders that were already on my computer, nothing to do with python or Anaconda. Is it safe to delete these folders from the iPython homepage?

Regarding the IPython Notebook, that's a great question. If your working directory when you launch the Notebook is your Desktop (for example), the Notebook interface will just show you what folders are on your Desktop. The reason it's showing you these folders is so that you can click on them to navigate to the location on your computer where you would like to store any new notebooks (or open existing ones). You can't delete those folders from the IPython Notebook interface, but you wouldn't want to even if you could!

In general, I would just recommend navigating via your command line interface (Git Bash, Terminal, etc) to where you would like to store notebooks, and then launch the IPython Notebook interface.

I was wondering if there's a better way to mapping categorical variables to numbers in Pandas? Something as simple as Factors in R? How would you transform the categorical response vector into a numpy array?

A couple notes:
1. I never use any spaces in Pandas column names, because then I can refer to the columns using "dot notation". For example: iris.sepal_length instead of iris['sepal_length'].
2. Rather than using a lambda function with the map method, you can actually just pass it a dictionary, which is what I did to convert the species to a number.
3. To answer your question, you can use scikit-learn's LabelEncoder to encode the species numerically. That's mainly useful when you have a lot of values to encode. I tend to use the map method when there are only a few possible values.
4. Although the X and y I created are actually DataFrames (rather than NumPy arrays), scikit-learn will handle the conversion to NumPy arrays. Thus, it's not strictly necessary to use ".values" when creating X and y.

Hope that is helpful! Don't hesitate to let me know if you have any questions or comments!